1. Technical Field
The present invention relates to a method of inspecting for the omission of a dot to be formed by ink landing on a medium in a printing apparatus such as an ink jet printer for intermittently discharging the ink onto the medium to cause the ink to land at target positions on the medium, and a printing apparatus for forming dots on a medium using the inspection method.
2. Related Art
As a printing apparatus, an ink jet printer for performing printing by intermittently discharging ink onto various types of media such as paper, fabric, or film is known. The ink jet printer forms an image by depositing fine dots formed of ink droplets on the medium. With regard to a configuration or printing operation of the ink jet printer, for example, the medium is moved in a particular transport direction and simultaneously, an ink discharge head in which nozzles for discharging ink over the width of the medium are fixed in lines or a zigzag form discharge ink while moving the medium in the transport direction, thereby forming an image. Accordingly, ink droplets discharged from the nozzles land at target positions on the surface of the medium. In addition, an ink jet printer of a so-called line printer type for forming landed ink droplets as dots to form an image, or a so-called serial printer for reciprocating a head in which nozzles are disposed in a range narrower than the width of a medium in a scanning direction intersecting the transport direction to land ink droplets discharged from the nozzles at target positions on the surface of the medium, are known.
Moreover, ink is charged in an ink tank, is led by a pump from the ink tank into a space called a reservoir inside the head, and then is guided from the reservoir to a pressure chamber connected to the nozzle. In addition, the pressure chamber is expanded or contracted to discharge the ink from the nozzle.
However, in the ink jet printer, there may be a case where the nozzles clog due to ink sticking and thus ink is not suitably discharged. In addition, there may be a case where the mechanism for discharging ink malfunctions and thus ink is not suitably discharged. In these cases, liquid droplets of the ink are not landed on the medium and a so-called “dot omission error” occurs. Therefore, in an ink jet printer, the existence of such a dot omission error needs to be inspected for. In this inspection, for example, dots are formed into a predetermined pattern on a medium and the dots are optically analyzed. The optical inspection may be performed on the predetermined pattern (test pattern) of the dots using an optical sensor or an image recognition technique, or may be performed by checking the test pattern with the naked eye. In either case, ink is actually discharged on the medium for the inspection.
Hitherto, for the ink jet printers, various methods of inspecting whether or not there is a dot omission error by actually performing printing on a recording sheet have been proposed. For example, in a method disclosed in JP-A-2005-35042, an image sensor is provided in the printer, and the printing state is detected using the image sensor to check for the existence of a dot omission. When the dot omission exists, a position of the dot omission is stored to be compensated for by another nozzle or the like during printing. In either case, there is a method of printing a predetermined pattern or the like as an inspection image and optically inspecting for a dot omission the inspection image using an optical instrument or with the naked eye.
In a general ink jet printer, to perform multi-color printing, ink is discharged from nozzles corresponding to respective colors to form a color image. In addition, as such color ink jet printers, there are various types depending on characteristics or kinds of ink used, methods of fixing ink droplets on a medium, and the like. It is determined that in certain types of printers, there are several cases where it is difficult to optically inspect for a dot omission, such as in a case where the ink color is similar to the color of the medium.